apply: carefully strdup a possibly-NULL name

2901bbe5 (apply: free patch->{def,old,new}_name fields, 2012-03-21)
cleaned up the memory management of filenames in the patches, but
forgot that find_name_traditional() can return NULL as a way of saying
"I couldn't find a name".

That NULL unfortunately gets passed into xstrdup() next, resulting in
a segfault.  Use null_strdup() so as to safely propagate the null,
which will let us emit the correct error message.

Reported-by: DevHC on #git
Signed-off-by: NThomas Rast <trast@inf.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -898,7 +898,7 @@ static void parse_traditional_patch(const char *first, const char *second, struc
 			patch->old_name = name;
 		} else {
 			patch->old_name = name;
-			patch->new_name = xstrdup(name);
+			patch->new_name = null_strdup(name);
 		}
 	}
 	if (!name)

t/t4111-apply-subdir.sh

@@ -86,6 +86,20 @@ test_expect_success 'apply --index from subdir of toplevel' '
 	test_cmp expected sub/dir/file
 '
 
+test_expect_success 'apply half-broken patch from subdir of toplevel' '
+	(
+		cd sub/dir &&
+		test_must_fail git apply <<-EOF
+		--- sub/dir/file
+		+++ sub/dir/file
+		@@ -1,0 +1,0 @@
+		--- file_in_root
+		+++ file_in_root
+		@@ -1,0 +1,0 @@
+		EOF
+	)
+'
+
 test_expect_success 'apply from .git dir' '
 	cp postimage expected &&
 	cp preimage .git/file &&